1. Field of the Invention
The present invention relates to an apparatus in the form of an air preheater for preheating the air for combustion for a combustion process by acting upon the air with hot, NO.sub.x -containing flue gases from the same or an independent combustion process; the NO.sub.x contained in the flue gases is simultaneously reduced.
So-called air preheaters are used to preheat the air for combustion for a combustion process. As a result of this preheating, there is achieved, among other things, advantages which relate to combustion and which result from higher combustion chamber temperatures, such as accelerated ignition and more complete combustion. Flue gas from the combustion process itself generally serves as the material flow which gives off heat for the combustion air which is to be heated up. However, it is also possible to use flue gas from an independent combustion process.
2. Description of Prior Art
A number of apparatuses for preheating the air for combustion are known. Such air preheaters include so-called recuperators and regenerators.
In a recuperative air preheater, the air is heated by the flue gas via heat transfer through solid walls. Preheaters of this type include, for example, plate or tube heat exchangers.
With regenerative air preheaters, the storage mass absorbs heat from the flue gas and later surrenders it to the air. For continuous operation, essentially two designs having cylindrical storage masses are encountered. One design has a rotating storage mass and stationary air and flue gas connections (Ljungstrom air preheater), and the other design has a stationary storage mass and a rotating air connection within the fixed flue gas connection which covers the entire storage mass (Rothemuhle air preheater). In both types of embodiments, the heat storage masses are designed as stationary bundles of laminations. With another type of heat storage mass, separate movable heat storage bodies are used in place of the fixed bundle of laminations; such a design is disclosed, for example, in U.S. Pat. No. 4,310,046.
In large furnaces, in addition to other noxious substances, there is produced NO.sub.x which pollutes the environment to a great extent. With regard to reducing the NO.sub.x emissions, a distinction is made between primary and secondary measures. The primary measures undertake action within the combustion chamber. Thus, a reduction of the NO.sub.x formation is already achieved during the combustion process, for example by the use of burners which are developed especially for this purpose, or by feeding the air for combustion in stages.
The secondary measures undertake proceduraI action outside of the furnace or boiler. Such action includes flue gas treatment processes according to which a reducing agent, such as NH.sub.3, is used in the presence of a catalyst, such as vanadium compounds on a titanium oxide base, with a reduction of NO.sub.x to molecular nitrogen and water vapor being effected. This process is carried out in a separate reactor which is provided for this purpose and is equipped with the catalyst. This reactor is generally disposed between the boiler unit and the air preheater. In many units, a high temperature electrostatic precipitator is disposed ahead of the reactor.
However, this known arrangement of the NO.sub.x reactor between the air preheater and the boiler unit has a number of drawbacks. On the one hand, there results a pressure loss of the flue gas as it flows through the catalyst bed. On the other hand, an appropriate amount of space has to be provided for this reactor. Furthermore, the support construction of the system has to be reinforced due to the weight of the NO.sub.x reactor. In addition, due to the dust content of the flue gases, the catalyzers have to be periodically cleaned, for example with soot blowers, which, of course, has a negative impact on the operating cost.
An object of the present invention is therefore to design an apparatus for reduction of the NO.sub.x from the flue gas while maintaining the effectiveness of the heretofore known reactors in such a way that the loss of pressure of the flue gas as well as the space required for the unit are minimized while at the same time reducing the energy requirement for the cleaning.